What are heating and cooling degree days?

Dear Tom,
What are heating and cooling degree days?
Colin McIntyre,  Chicago
Dear Colin,
Heating and cooling degree days are measures that indicate the departure of a day’s average temperature from a base of 65 degrees. When the day’s average temp (the sum of the midnight-to-midnight, CST, high and low temperatures, divided by two) is below 65 degrees, the number of degrees of departure is called heating degree days; when above, cooling degree days.

Degree day units are a useful guide for operations in which temperature is an important factor — in heating and air conditioning, for example, and in agriculture. A base of 65 degrees is used because heating engineers have determined that when air temperatures average below 65 degrees, heating is generally required in a building; when above, air conditioning is helpful.

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